Pacing leads having at least two electrodes adjacent to the distal end are used for bipolar pacing, as well as for providing sensory information to an attached cardiac pacemaker. Generally, bipolar pacing leads are either active or passive fixation, the former using tines typically, and the latter using a screw mechanism. A bipolar active fixation lead can have its helical screw be one of its electrodes and a ring electrode spaced from the distal end of the pacing lead as its second electrode. The spacing between the two electrodes is usually dependent upon the physical constraints attendant with the design of the distal end of the lead and its materials. However, it is beneficial to have the two electrodes very close together, particularly for sensing differences in the electrical signals in the heart.
The design and proximity of the electrodes can be such as to enhance the electrical signal sensing capability of the bipolar lead to allow improved discrimination of the sensed signals. Generally, abnormal electrical activity such as ventricular tachycardia or ventricular fibrillation results in electrical signals having differing characteristics. For example, in a given patient, ventricular tachycardia may produce electrical potentials much higher than normal sinus rhythm. By comparison, ventricular fibrillation may produce electrical potentials which are smaller than that of normal sinus rhythm signals. Similarly, in the atrium, electrical potentials also vary whether the signals are normal sinus rhythm signals or due to atrial tachycardia or fibrillation. The ability of a pacing lead to discriminate between the signals is at least partially dependent upon the spacing between the two electrodes. The ability to sense the electrical potential across a small area within the myocardial tissue would be very beneficial in allowing a pacing system to discriminate between the various electrical signals within the heart. In addition, minimizing the spacing between the electrodes used for bipolar sensing would also minimize the sensing of “far field” electrical signals generated elsewhere in the heart or from, for example, nearby skeletal muscles.
Typical of the known prior art is U.S. Pat. No. 4,010,758 to Rockland et al. which discloses a bipolar electrode structure comprising a first, helix-configured electrode adapted to be implanted within the body tissue, for example, the heart, by rotation or screwing and a second, annularly-shaped electrode disposed substantially concentric about the first electrode upon the surface of the tissue, for example, the epicardium.
Another instance of the prior art is found in U.S. Pat. No. 5,545,201 to Helland et al. that discloses an implantable bipolar pacing lead having a bipolar active fixation electrode for use with a cardiac pacemaker. The bipolar active fixation electrode may include a pair of coaxial electrodes, separated by an intermediate insulator, formed into the shape of a helix. The bipolar electrode helix is preferably advanceable from a distal end of the bipolar pacing lead. At the tip of the bipolar electrode helix, the outer electrode is removed, so that the intermediate insulator and the inner electrode extend from the outer electrode, spacing the two electrodes a distance of between 0.1 and 5.0 mm.
U.S. Pat. No. 6,397,109 discloses an implantable lead system that includes a coronary sinus stent supporting multiple electrodes, the stent structure kept in its compressed form while introduced into the access vein, then expanded, dilating a bifurcation of the lead and pushing the electrodes against the cardiac walls. A suitable pre-curvature is imparted to the lead body along its length to enable ease of positioning of the lead structure in the coronary sinus.
It was in light of the foregoing that the present invention was conceived and has now been reduced to practice.